One of the KDE4 features was the possibility to bind app to single activity. In Plasma 5.2 beta, if we click on window decoration and pick More Actions -> Additional Application Settings and open tab "Size&position" than select Activities --> Force (from dropdown) and open next dropdown, we can see only option "All activities" and an empty one but all existing activities should be listed here. I have 3 activities defined and none of them is listed there.
I can confirm this issue on d00d3ea, kactivities on 36d695f
CCing Ivan Čukić..
This is fixed for 5.4